This invention relates to highly-oriented materials and to processes for nucleation and growth thereof and, in particular, in a process involving the deposition of a crystal structure on a substrate, to the improvement for stimulating crystal growth comprising the step of providing highly-oriented poly(tetrafluoroethylene) having an orientation angle of less than 20.degree. on the surface of the substrate. More particularly, the method includes provision for stimulating highly-oriented crystal growth along a pre-established orientation director by additionally comprising the step of orienting the poly(tetrafluoroethylene) with its nominal orientation director along a desired orientation director.
The formation of highly-oriented structures, such as single crystals, single-domain liquid crystals, or uniaxially-oriented crystallites, is of major technological importance for many applications that range from reinforcing materials to electronic displays. Oriented materials can be manufactured in many ways; however, general techniques to orient a great variety of different materials are not available. Currently, the selection of the appropriate method is governed primarily by the nature of the species that is to be oriented. Moreover, techniques for stimulating crystal growth, particularly of a highly-oriented variety, are often difficult and very expensive to implement in an industrial environment. As a result, the associated products are of higher cost then they would otherwise be.
Small molecules are frequently processed into oriented structures by the tedious growth of single crystals, or by a technique known as epitaxial crystallization, onto single-crystalline substrates. Regarding the latter method, it is generally believed by those skilled in the art that lattice matching of the crystalline substrate and material of interest is required in two dimensions for epitaxial crystallization to occur. As a result, relatively few systems are known in the art; and, in many cases, new epitaxial surfaces need to be found for each newly discovered material. Examples of substrates commonly used for epitaxial growth are quartz, muskovit, alkali halides, and salts of aromatic organic acids. More recently, a technique referred to as graphoepitaxy has been proposed for the oriented growth of some materials. In this method, materials are deposited on substrates (frequently amorphous) onto which a precision relief grating has been etched. Typical grating distances are of the order of 0.3 .mu.m. According to reports, graphoepitaxy has been successfully applied to some metal halides and SiO.sub.2.
The use of some moderately-oriented, semi-crystalline polymers as substrates for epitaxial growth of a few small molecular materials has been described in the art. For example, moderately-oriented polyethylene, isotactic polypropylene, polybutyl, and isotactic polystyrene have been used as substrates for epitaxial crystallization of several metals. Although some success was reported for a few species, such as Sn, Te and Bi, the phenomenon certainly was not general. For example, no effects were observed with Zn, Au, Ni, Co, Ga, and Sb. No epitaxial effects were observed with poly(vinylidene fluoride) substrates, and the use of poly(tetrafluoroethylene) was not described or suggested.
Amphiphilic molecules generally are oriented into structures known as Langmuir-Blodget films. This technique becomes tedious and ineffective for thicknesses beyond a few monolayers and is, therefore, currently of limited practical use.
Liquid crystalline materials of small molecules are generally oriented in electric or magnetic fields. Another method for orienting liquid crystalline materials consists of the deposition onto "rubbed polymer surfaces", which appears to resemble a somewhat rudimentary form of the graphoepitaxial method mentioned above. Also, surfaces onto which SiO is obliquely evaporated have been used to align liquid crystals.
Oriented structures of macromolecular materials generally are made by expanding melt- or solution- processed, or paste-extruded and/or sintered objects, such as films, tapes and fibers, through well-known drawing technologies. These techniques are routinely applied for many polymeric materials. Examples of such practices are well-known in the art, and are disclosed in, for example, U.S. Pat. Nos. 2,776,465 and 3,953,566. Mostly, the drawing methods are limited to maximum draw ratios (final length/original length) of 10 or less, which leads to the formation of macroscopic structures of only moderate orientation, and of limited length. Generally, the degree of orientation achieved in tensile drawing is moderate and can sometimes be inhomogeneous. Only recently, methods have been disclosed to achieve draw ratios of 100 or more; but, these techniques are applicable to only few high polymer systems. Examples are disclosed in U.S. Pat. Nos. 4,344,908, 4,411,854 and 4,769,433.
A few reports have been published on the orientation of polymers through epitaxial crystallization onto small molecular crystals; however, as was observed also for small molecular epitaxy, only a few selected systems were found to be effective, and the method is not used commercially. An attempt was reported by Takahashi, et al. to epitaxially crystallize polymers onto tapes of poly(tetrafluoroethylene) that were moderately-oriented by drawing to four times their original length. Such drawn polymer tapes have orientation angles, as measured by wide-angle X-ray scattering, of 30.degree. or more. Only very weak orientation was observed for polyethylene, poly-.epsilon. caprolactone and nylon 6 that were crystallized onto these poly(tetrafluoroethylene) tapes. The growth of other materials, notably liquid crystals, small organic molecules, and oligomers and other polymers was not disclosed or suggested.
The prior art orientation methods described above are most certainly not generally applicable to a wide variety of materials; and, in addition, mostly yield relatively poor orientation on a macroscopic scale. The crystal size of the epitaxially crystallized materials of interest, in many instances, is very small, typically in the nanometer-micrometer range.
It is thus an object of this invention to provide oriented poly(tetrafluoroethylene) structures as substrates for the formation of macroscopically-oriented structures of a vast variety of materials.
More specifically, it is an object of the present invention to provide orientation inducing substrates and a method for inducing the formation of macroscopic, highly-oriented structures of a wide variety of materials.
Other objects and benefits of the invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.